VOLCANIC ASH OF HAWAII AND ITS SOURCE.
The district of Kau between Puna and Kona is proverbially dusty. The floor is of modern lava, covered over an area of three hundred square miles with a light yellowish dust. Mountain torrents have washed away some of it, revealing basalts just be ginning to disintegrate ; that which remains is very loose, easily moved by wind or water. In the older days the natives enjoyed jumping from a high bank into the dust, just as they might leap from a bluff into the water. Of course this material is badly cut down by teams along the roads. It is utilized for the growth of sugar cane everywhere that plantations exist on the west side of Kilauea. These soils are free from rocks and are very deep, so that a crowbar or cane may be readily thrust down its whole length, just as would be true of large piles of wood ashes in a dry country. Neither is there anything adhesive in this dust when wet. No part of it adheres to one's shoes when walking over it in time of rain.
These soils suffer badly from drought. Extensive fields will be parched and clouds of dust will be very annoying, even im parting a reddish yellow tint to the sky. When the rain comes in torrents much damage will be done to the land by the cutting of trenches and the transportation of earth. The dry and wet periods are registered in the varied and irregular length and dia meter of the joints of the sugar cane stalks. In the season of drought much pains are taken to prevent the starting of fire in the grass, as it spreads long distances beneath the surface, be cause the spongy nature of this ash will allow the access of air to support the combustion.
It is often dangerous to traverse the forests above the planta tions on horseback, because the animals unexpectedly plunge into unseen deep holes and break their legs. Surveyors find it im practicable to carry supplies to their workmen by direct routes over these soils and necessarily make wide detours.
In traveling from Kilauea southwesterly through Kau this ash first appears in small isolated areas four miles from the volcano, and then increases in amount and importance, and is more notice able about the "Halfway House." Between this and Pahala cer tain piles of it, as at the level of 1,800 feet, resemble terraces. It is the material supporting the Pahala sugar plantations. It has been covered at various places in Kau by flows of pahoehoe. An isolated bill of this sort near the tramway a mile or more northeast from. Punaluu harbor is conspicuous. As a rule, the lands near the sea level have either lost this ash by rain erosion or it is covered by the later lava flows. Most of the peaks in the Mohokea area are capped by the ash, though it is recognized most abundantly near the southeast margin.
The promontory called Kahuku Point, South Cape, and Ka Lae is likewise covered by this ash, and has attained the thick ness of ten feet, separated into two parts by a thin seam of earth.
The late eruptions of 1868 and 1887 destroyed the continuity of this deposit between Kahuku and Kona.
Mr. Emerson has discussed the problem of the source of the aerial eruption, and the writer has referred to the same question ni a paper on the volcanic phenomena in Hawaii." King Umi's road is referred to as giving evidence of the pres ence of these ashes for three and a half centuries. He occupied a tract of land between Mauna Loa and Hualalai, where some of the edifices constructed by him were figured by Captain Wilkes and are still to be seen. The road ran north and south, parallel to the shore of Kona, seven or eight miles distant, to a natural amphitheater on the southern slope of Puu o Keokeo, where im mense crowds of Hawaiians gathered to witness the cock fights. The pens still stand as they were in Umi's day. The road over this ash is said to be only two or three feet wide. If a mule traversing this path deviated but a few feet on either side he would sink down to his girth and flounder helplessly. If a shower of pumice or lapilli had fallen since the days of Umi, the road and the pens would have been swept away or covered up. Hence we must regard the ash deposit as the latest formation of the neighborhood, though still several centuries old.
Mr. Emerson's final conclusion is that we must seek for the source of the ash in the district where it abounds. Considering the shape of our supposed caldera, he thinks the ashes must have proceeded from some part of it. This was the "source of the stupendous explosions or series of explosions which has rescued Kau from being a waste of unproductive rock and transformed it to so large an extent into a land of pastures and plantations." I have already treated of this question in the paper cited, look ing to Mokuaweoweo as the probable source of this and other localities of ash on Hawaii. What is conceived to be the same duplex deposit is recognized at Puakala on the south flank of Mauna Kea, at Hilo, all through Olaa, as well as in Kau and Kona. I have also discovered the same deposit on the north side of Mokuaweoweo a dozen miles west of Humuula sheep station, so that now the great crater has been proved to be encircled by this light, fine-grained material. The absence of it about Kilauea, Puu o Keokeo, and on the north slope of Mauna Loa is occa sioned by its removal by the later historic discharges of lava. It would not be found near the central vent because the heated air would carry the particles many thousand feet in the air, whence they would descend miles away from their place of origin. The fact that the Mohokea caldera is covered by the ashes is evidence that they came from a distant vent. Had the eruption been in the midst of the depression, we should look for them in an en circling belt, if not upon the southwest side almost exclusively, where they were deflected by the trade winds.